Studies were directed toward determining the steps involved in the differentiation of B cells into immunoglobulin synthesizing and secreting plasma cells. Special emphasis was laid on developing new techniques to study the role of helper T cells, macrophages and suppressor cells in these immune regulatory process and to define defects in these immunoregulatory cell interactions in patients with immune dysfunctions. Leukemias of both suppressor T cells have been demonstrated in association with agammaglobulinemia, selective IgA deficiency, suppressor leukemias and infectious mononucleosis. Excessive numbers of non-T cell suppressors have been demonstrated in multiple myeloma associated with immunodeficiency. Although T cells are not the effectors of suppression in multiple myeloma they are required elements in the chain of cells required for the generation of the suppressor monocytes.